Low carbon steel



November 18, 1926.

Patented Apr. 19, 1932 PATENT OFFICE ADOLI FRY, OI EBSEN,

GERMANY, ASSIGNOR TO FRIED. KRUPP AKTIENGESELLSCHAIT,

0F ESSEN-ON-THE-BUHR, GERMANY LOW CARBON STEEL No Drawing. Applicationfiled December 16, 1927, Serial No. 240,649, and in Germany August 27,1820.

- This application isa continuation in part of my application Serial No.149,280, filed Articles made of low carbon steel suffer from thedrawback, in particular when in aged condition, of becoming brittle,when exposedto the efi'ect of liquids, vapors and gases which may causea feeble reaction, for instance hot caustic soda solution of 45 percent, alkaline boiler feed water or ammonia gases. My invention has forits object to obviate this drawback. a

According to the invention this object is obtained by employing for themanufacture of said articles a low carbon steel of a content of carbonup to 0.4 per cent and in which the substances tending to segregatebetween the grain boundaries, such as oxygen, oxygen compounds, sulphur,carbon etc., are prevented from doing so.

The reason why the said drawback of becoming brittle is obviated bypreventing these substances from segregating between the grainboundaries, may be as follows:

The substances generally present in low carbon steel, such as oxygen,oxygen compounds, sulphur, carbon eto., have the tendency to segregatebetween the grain bound-' aries, where they form either an intermediatelayer separating the individual grains, or small nests, which in itselfdecreases the tenacity of the material to some extent. Now, when such asteel, in particular when in aged or tensioned. condition, is exposedfor instance to the eflect of a hot caustic soda soluprevailing betweenthe individual grains is lessened. This process results in the greatdecrease of the tenacitymeasured by a bending test of a notched bar ofthe steel-which is feared in the art as the so-calledsolutionbrittleness and appears after a relatively short time. A similarphenomenon arises when the steel is exposed to the effect of vapors orgases. *If, for example, the effect is produced by hot hydrogen gas atabout 500 C. and' under high pressure, the hydrogen penetrating alongthe grain boundcavities to be produced in the texture. This phenomenonlikewise results in the steel becoming brittle very quickly. A similarbrittleness is caused when sulphur which mayexist in the grainboundaries, is dissolved by the effect of vapors or gases.

The result common to all these cases is that a brittleness of the steelis developed, instead of the said originally prevailing normaltenactity, and that this brittleness is caused by removing substancesfrom the grain boundaries of the steel only when the articles have comeinto use. If, however, the substances mentioned are prevented fromsegregating between the grain boundaries, or removed therefrom from thebeginning, that is before the articles are used, for instance in themanner hereinafter proposed, a much more intimate coherence of thegrains will prevail, as this coherence is not disturbed by any noxiousintermediatelayer or nests. Consequently, the possibility that theliquids, vapors and gases in consideration will penetrate along thegrain boundaries into the steel and thus lessen the coherence of thegrains is; very much reduced or totally removed. Hence, the originallyprevailing tenacity of the bent notched bar cannot be substantiallydecreased, or decreased at all, even after a long action of saidliquids, vapors and gases upon the steel.

In the case of steel which has already become solid and wherein,therefore, the substances mentioned have already segregated between thegrain boundaries, then by heating the steel at a temperature above about650 C. and subsequently quenching it with or without reheating up to atemperature not.

exceeding 750? 0., the said substances are made to enter into solidsolution with the steel and are held in this condition. In par- Now,when the said substances are most finely divided by quenching from thestate of solid solution or the carbon is in the condition of grainedcarbide, so that the grain boundaries are free from them, thereis'practically no possibility that the liquids, vapors and gases will becapable of dissolving said substancesout of the texture when acting uponthe steel, and thus of causing the brittleness.

It may be left undecided how far the explanations made hereinbefore willstand firm in detail against scientific investigation. In every case,however, it clearly appeared from exact experiments, that test pieces ofa steel treated in the indicated way, showed their original tenacityafter having been exposed for a long time to the attack of hot causticsoda solution of 415 per cent. Other test pieces of the samecomposition, but not subjected to a previous treatment according to myinvention, were destroyed in a short time by cracks, after having beenexposed in a similar manner to the attack of the soda solution.

What I claim as my invention and desire to secure by Letters Patent, is

1. Articles which must be capable of withstanding the corrosive efi'ectof mildly corrosive liquids, gases, and vapors, characterized by thefact that they are composed of a steel containing an amount of carbonnot more than 0.4%, which steel has been quenched from a temperature of650 to 950 C. i

2. Articles which must be capable of withstanding the corrosive effectof mildly corrosive liquids, gases, and vapors, characterized by thefact that they are composed of a lowcarbon steel which has been quenchedfrom a temperature of 650 to 950 C. and

subsequently drawn at a temperature not exceeding 750 C.

3. Articles which must be capable of with! standing the corrosive effectof mildly corrosive liquids, gases, and vapors, character ized by thefact that they are made of a low carbon steel which has been heated to atemperature of about 7 30 C.

The foregoing specification signed at Cologne, Germany, this 30th day ofNovem ber, 1927.

. ADOLF FRY.

